Pauli gates
The three basic single-qubit operations X, Y and Z — half-turns about their respective axes on the Bloch sphere.
X is the quantum NOT: it swaps zero and one. Z leaves the probabilities untouched but flips the sign of the phase, which is invisible when measuring a lone qubit yet decisive in interference. Y is the combination of both.
The gates are named after Wolfgang Pauli, whose 1920s matrices describe electron spin. Beyond being algorithmic building blocks, they are the language in which errors are described: any error on a qubit can be expressed as a bit flip, a phase flip, or both. That insight is what makes quantum error correction possible despite errors being continuous.